Natural rubber latex gloves provide excellent protection from numerous dangerous pathogens as well as many harsh chemicals. The natural rubber latex glove manufacturing industry mushroomed in early 1980s, especially in the Far East. However, soon after that, it was recognized that the inherent proteins of natural rubber latex would cause allergic reactions (Type I) to occur in certain people. In rare cases, the allergic reaction could be fatal. Therefore, for those people, alternatives, to natural rubber latex gloves, must be provided.
Although a series of synthetic materials including nitrile butadiene rubber (NBR), polychloroprene (CR), polyurethane (PU), polyisoprene (IR), polyvinyl chloride (vinyl, PVC), polyethylene (PE), etc. as well as many of their blends and copolymers have been used as alternatives to natural rubber latex, the overall performance and the cost of the alternatives are not quite satisfactory. Among the alternatives, nitrile butadiene rubber is the most popular one, an elastic glove at a reasonable cost.
U.S. Pat. No. 5,014,312, and Reissue Patent RE 35,616, both issued to Tillotson et al, cover nitrile butadiene rubber gloves. The patents address relaxation properties. The stress (or modulus) of the material under constant strain at six minutes should be less than 50% of its initial value. Most of the nitrile gloves currently commercially available have their relaxation property clustered about 40%, although that could be varying from 30xcx9c45%. Other gloves might have improved tensile strength, or elongation, or fewer additives that could cause Type IV allergic reactions (ZnO, etc.). None of them have displayed relaxations at six minutes that could exceed 50%.
Relaxation property is not an ASTM required quality control parameter for gloves. But together with modulus, another non-ASTM required quality control parameter for gloves; they can characterize the performance and the tactile sensation of, a glove. The higher the relaxation property, the better the glove will fit a hand""s shape. Otherwise, the glove becomes loose after a while. But if high relaxation is combined with high modulus, the glove would quickly cause finger fatigue. Natural rubber latex gloves has a (relaxation greater than 80%, and a 300% modulus ( less than 2 MPa.), while nitrile butadiene rubber gloves show lower relaxation (typical 40%) and a much higher modulus at 300% ( greater than 7 Mpa).
Relaxation property is an intrinsic characteristic of material nature. Most nitrile butadiene latexes manufactured via emulsion polymerization would yield a relaxation of about 40%, as evidenced by the nitrile gloves currently available. This inherent property is predominantly caused by polymer chain structure, which would be determined by the polymerization mechanism. Different nitrile butadiene rubber vendors might have different controlling parameters and procedures, but their products have very little differentiation due to the fact that they all use emulsion polymerization for economic reasons.
Nitrile butadiene latexes, produced via polymerization mechanisms other than emulsification, namely for dipping. applications could have quite different structure, and thus different relaxation profiles, but there are no such products that are commercially available right now because of cost. Once the polymer chain structure has been predefined in the polymerization, there is little one can do to manipulate it. It is an objective of the invention to tune this parameter (relaxation) to above 50%. Meanwhile, the other mechanical properties must meet ASTM requirements.
This invention is a combination of an unique zinc oxide free formulation and selected vulcanization conditions that can be achieved economically with common production facilities. Produced are gloves that have a relaxation property, higher than 50%, and a low modulus (approximately 3 Mpa). The glove maintains decent ultimate tensile strength ( greater than 20 Mpa) and elongation ( greater than 500%). The glove is produced by a vulcanization process which lasts from 5 to 60 minutes at temperatures ranging from 300xc2x0 F to 400xc2x0 F. The tensile strength and elongation are well above the ASTM requirements for medical gloves. The current ASTM requirements are ASTM D412-92. Thanks to sufficient vulcanization, the films produced provide satisfactory protection from viral penetration. The tearing strength is also better because of the lower modulus.